The prior art is replete with numerous examples of hot melt adhesives which are employed for the construction of disposable soft goods. Specific applications for these prior art adhesives have included disposable diapers, sanitary napkins, surgical drapes, hospital pads and adult incontinent products to name but a few. Moreover, the prior art methods of application of these prior art adhesives have included, but are not limited to, extrusion (multi-bead or slot), and spray or wheel application systems.
Those skilled in the art will readily recognize that many different polymer bases have been used, heretofore, to formulate hot melt adhesives for the construction of disposable soft goods. In this regard, the first copolymers to be employed were the ethylene vinyl acetate copolymers (EVA) and amorphous polypropylene (APP). While these polymers, when properly blended, provided acceptable adhesion to most substrates, they had several shortcomings which detracted from their usefulness. One of the first shortcomings of these polymers was that they lacked the desired elevated temperature resistance. For example, it is very important that a construction adhesive, for disposable soft goods, maintain its bond, not only at room temperature, but also at elevated temperatures, that is, 100.degree. F., 38.degree. C. This elevated temperature resistance is important because without this characteristic, delamination of the end product occurs if the adhesive bond comes into contact with the user's skin. A second shortcoming of the prior art adhesives which were based in whole or in part on EVA or APP is that these polymers have a tendency to "gel" or otherwise increase in viscosity, or char when subjected to typical commercial application temperatures, that is 300.degree.-350.degree. F. In most instances, this drawback manifests itself in the form of poor application characteristics, such as plugged equipment nozzles. Further, those skilled in the art will recognize that adhesives based upon EVA or APP cannot generally be formulated as multi-purpose adhesive compositions.
As should be understood, multi-purpose adhesive compositions are those adhesives which can be used for more than one application. For example, and in the manufacture of most disposable diapers, today, it should be understood that several different adhesive applications are present. These adhesive applications include, the use of adhesives in construction, that is, bonding the polyethylene to the nonwoven and absorbent pad; on some diapers this adhesive also bonds a colored film in place at the end seals; the use of adhesives for elastic attachment, that is, bonding the elastic material to the polyethylene in either the leg and/or waist area; the use of adhesives for landing strips, that is, bonding a reinforcing layer of polyolefin film to the polyethylene in the area opposite the tape tabs; and the use of core adhesives, that is, applying an adhesive to the absorbent core to increase the strength of the core.
As should be appreciated, multi-purpose adhesive products which can be used for more than one application are desireable inasmuch as they reduce the number of different adhesive products which must be held in a manufacturers inventory. Furthermore, a reduction in the number of adhesive products employed by a manufacturer reduces the likelihood of the wrong adhesive being used during a manufacturing cycle. In addition to the foregoing, the use of a multi-purpose adhesive will, as a general manner, be less costly because of the volume discounts which are generally available when larger quantities of a single adhesive product are purchased.
As discussed earlier, the prior art EVA and APP based adhesive compositions have not generally been compounded, heretofore, to form multi-purpose adhesives with pressure sensitive properties because these formulations were found to be severely compromised as to the strength, and elevated temperature resistance. As should be understood, pressure sensitivity is extremely important to multi-purpose adhesive compositions because a pressure sensitive adhesive will form a bond over a much wider range of application conditions, such as temperature, than a non pressure sensitive adhesive composition.
In addition to the prior art adhesives discussed above, adhesives based on styrene-butadiene-styrene (SBS) block copolymers were suggested for use in the construction of disposable soft goods. An example of these prior art adhesives is disclosed in U.S. Pat. No. 4,526,577. The SBS based adhesive compositions appeared to be improvements over the previously employed adhesives in several important respects, but they also had several drawbacks which also detracted from their usefulness. For example, it was discovered that when an SBS based adhesive composition was left in an adhesive applicator for an extended period of time, it would rapidly increase in viscosity and ultimately gel thereby making it extremely difficult to remove. Furthermore, adhesive compositions based upon SBS copolymers did not appear to have sufficient elevated temperature creep resistance to perform well as an elastic attachment adhesive as compared with adhesive compositions based upon the styrene-isoprene-styrene (SIS) block copolymers of the present invention. Furthermore, many diaper manufacturers have recently added a foamed elastic waistband to their disposable diaper products, and the SBS based adhesive compositions appear completely unacceptable for this particular manufacturing application. This appears to be due to insufficient elevated temperature creep resistance.
Recently, many disposable garment manufacturers have begun to use spray application equipment. In this particular manufacturing equipment, it should be understood that immediately after the adhesive composition is extruded through a nozzle, it is picked up by an air stream which transports the adhesive composition to the desired substrate. As should be understood, this method of applying an adhesive tends to reduce the temperature of the adhesive composition, when compared to other application systems, even if the air stream is heated. In addition to the foregoing, the adhesive composition is substantially elongated by this process, which further increases the cooling effect because the adhesive composition increases in its overall surface area. In some applications, this cooling effect can be exploited. For example, this cooling effect, in spray application systems, can be used to bond substrates together which are very heat sensitive. However, and because of the cooling effect which takes place, it is usually necessary that the adhesive composition have a high degree of pressure sensitivity even when they are cooled by this type of application equipment.
As discussed above, adhesive compositions based upon styrene-isoprene-styrene (SIS) block copolymers have also been used, heretofore, in the construction of disposable soft articles. While these adhesive compositions have been used, they also have had several noteworthy deficiencies which have detracted from their usefulness. For example, adhesive compositions employing previously commercially available SIS copolymers displayed a low modulus and poor elevated temperature resistance even when formulated with various tackifying resins. In an attempt to improve the temperature resistance of the various SIS based compositions, end block reinforcing resins were blended with same. However, these resins appeared to decrease the specific adhesion of the adhesive compositions to polyolefin substrates and also raised the raw material cost of the final adhesive composition inasmuch as these reinforcing resins are generally quite expensive. Moreover, it appeared that, with respect to maintaining any significant degree of elevated temperature resistance, adhesive compositions containing SIS copolymers appeared to require relatively nonpolar tackifying resins, that is, partially or totally hydrogenated resins or aliphatic C-5 resins. However, it became evident following experimentation that resins containing a significant amount of polar or aromatic components tended to associate with the end block of the block copolymer, and thereby decreased the elevated temperature resistance to unacceptable levels. As a result, and when adhesive compositions were compounded using SIS copolymers and these relatively non-functional resins, the resultant adhesive compositions had what was considered, very poor adhesion to polyolefin substrates. While these same adhesive compositions were acceptable for use as elastic attachment adhesives, that is, where a high level of specific adhesion is not generally needed, they further appeared completely unsuitable for use as construction adhesives, that is, where a very high level of specific adhesion to polyolefin substrates was mandatory. Moreover, these same compounds tended to yield products with undesireably high viscosities at application temperatures. As a result of the foregoing, it was believed that adhesive compositions based upon SIS copolymers would not be suitable for use as multi-purpose adhesives.